Incandescent light sources produce a near black-body optical spectrum. However, such light sources tend to have relatively low efficiency, and are relatively large requiring large light fittings, particularly if a low divergence angle is required for the light output. High intensity discharge lamps are capable of producing high luminous flux from small arc sources. Such sources are suitable for projectors and directional illumination but are bulky and have relatively low efficiency. Fluorescent lamps have improved efficiency compared to incandescent sources, but suffer from a large source size.
Light-emitting diodes (LEDs) formed on monolithic wafers (wafers consisting of a single piece) can demonstrate high efficiency with acceptable CIE Color Rendering Indices (CRI). Organic light-emitting Diodes (OLEDs) can be formed on arbitrarily large substrates but may have lower optical output per unit area compared to wafer type LEDs, which may limit their efficiency in directional lighting.
Illumination apparatus, including for example (but not limited to) ceiling lights, wall lights, cabinet lights, torches, automotive lights, architectural lights, medical lights and so on, is used to illuminate an area or a volume, so that the light can illuminate items in that area or volume, so that those items can be viewed by a person. This is distinct from display technology e.g. three dimensional (3-D) display technology and in particular autostereoscopic 3-D display technology (in which no viewing aids are required), where light is directed from display elements to different eyes so that the display elements themselves can be seen directly by a person.
In lighting applications, light from an emitter is directed using a luminaire to provide the required directionality. The angular variation of intensity (or luminance) is termed the directional distribution, which produces a light radiation (illuminance) pattern, or ‘spot’ on surfaces and is configured for a particular application. Lambertian emitters enable light to flood a room with light. Such use of light can be inefficient as the light is distributed to regions that may not require illumination, and can be perceived as a wasteful and visually undesirable illumination source. Lambertian emitters can be perceived as providing flat lighting lacking in sparkle which is unattractive. Further, Lambertian emitters produce high levels of source glare, so that an observer will see the light source from a wide range of angles with high luminance, providing visual discomfort.
Non-Lambertian, directional light sources use a relatively small source size lamp such as a tungsten halogen type in a reflector and/or reflective tube luminaire, in order to provide a more directed source. Such lamps enable efficient use of the light by directing it to areas of importance. These lamps produce higher levels of visual sparkle, in which the small source provides specular reflection artifacts, giving a more attractive illumination. Further, the source glare can be reduced, as the output aperture of the light source is directly visible only from a small range of angles.
To further enhance usage efficiency, it would be desirable to adjust the directionality of directional light sources to meet the lighting requirements of an environment. For conventional directional lamps, this is typically determined by the design of the reflector optical elements and the pointing direction of the lamp during installation. An adjustment of pointing direction often adds cost requiring gimbal type mounts and can often be difficult to adjust because of the location and surface temperature of the lamp. The directionality of the lamp is typically set once by design choice. To deal with the changing usage patterns in the room, multiple directional lamps are installed giving an inefficient over-illumination of a room. For example, it might be advantageous to use broad Lambertian illumination in the daytime to supplement natural lighting, while in the evenings, it might be preferred to use more directional spot lighting. In another example, it may be desirable to achieve more rapidly time varying lighting effects by varying the structure of arrays of lights illuminating a surface so as to provide visual impact.
Directional LED elements can use catadioptric optic type reflectors combining refractive and total internal reflector elements, as described for example in U.S. Pat. No. 6,547,423. A known catadioptric optic system is capable of producing a light beam with a 6 degree divergence angle from a 1×1 mm light source, with an optical element with 15 mm final output diameter. The increase in source size arises for conservation of brightness (etendue) reasons. This element provides optimum efficiency for an LED positioned in the input aperture of the device. Adjusting the separation of the LED from the input aperture may change the output divergence angle somewhat, but can also reduce collection efficiency. Further, this may require mechanical adjustments that are difficult to control at low cost and are unreliable over the lifetime of the lamp.
It is therefore desirable to add a means to adjust the directionality of the light in response to an applied electrical signal. This would achieve more efficient illumination of an environment to suit the needs of users.
Electrically switchable birefringment liquid crystal microlenses are described in European Optical Society Topical Meetings Digest Series: 13, 15-16 May 1997 L. G. Commander et al “Electrode designs for tunable microlenses” pp 48-58. Circular liquid crystal microlenses provide variable directionality in two axes but are difficult to provide with uniform alignment orientation by conventional alignment layer processing methods. Further, such lenses suffer from liquid crystal alignment disclinations which can degrade the performance of the lens in switched and unswitched states.
Switchable 2D-3D displays using a switchable lenticular lens array are described in WO 98/21620, WO 03/015424, WO2004/070467, WO2005/006774, and US20070008617. Autostereoscopic displays comprising switchable lenticular arrays image arrays of pixels to viewing windows. An observer positions their eyes within the viewing window and looks directly at the autostereoscopic display. In the case of lenticular displays, the observer thus looks directly at the lenticular elements. The switchable lenticular elements are switchable between a 2D mode in which the full panel resolution can be observed in which no autostereoscopic viewing windows are produced and a 3D mode in which the autostereoscopic viewing windows are produced. In an illumination system, an observer does not look directly at a light source due to its high brightness; rather the observer sees light reflected and scattered from other surfaces. US20080284924 describes a projection apparatus for a lighting system comprising at least one liquid crystal optical element.
Switchable diffusers using polymer dispersed liquid crystals encapsulated in polymeric material are known. Such elements typically suffer from high levels of back scatter and thus low optical efficiency and are not suited to efficient lighting systems with controllable directionality.